Acoustic apparatus



June 4, 1935. c, LEMENT 2,003,284

' ACOUSTIC APPARATUS Filed Nov. 19, 1928 ATTOP/V ers and more in particular to hydrophones for indeed, many other uses which *j 'a combination is,v

Patented June 4, 1935 7 UNITED STATE Aoous'rro ArrAnA'rUs corporation of Maine Ivan C. Clement, Greenwood, Mass Submarine Signal Company, Boston,

assignor to Masa, a

Application November 1 9, 1928, Serial No. 320,368

. 3 Claims. The present invention relates to sound receivreceiving sound in'yvater.

For many purposes it is desirable to have a sound receiver which will respond with equal intensity over a broad range of frequencies.

Such a receiver is'useful in listening tonoises of a boat in the water where there is no single pitch of the sound but a general mingling of many sounds of different frequencies and char acteristics. It is also useful in the transmission of speech under water-and in the air. It has,

will not be enumerated here,

Receivers of this character have been built, some of which. have been fairly successful and others of which have failed in operation in many places where it has been attempted to use .them. One type of receiver-which has been used combines the balanced armature drive or detector unit with a soft rubber diaphragm.

While this type of receiver has proved to be better than the earlier types used, nevertheless the rubber diaphragm is not still enough to make a sensitive receiver, since it has a tendency to yield to the sound vibrations impinging on it and passing through it without imparting a definite amplitude to the detecting unit. In other words, not all the energy picked up by such a diaphragm isavailable to operate the detector unit. Such therefore, rather insensitive. Then again, when used in deep water the static pressure on the diaphragm is sometimes sufiicient to alter the diaphragm characteristics or its physical relation to the pick up unit, bringing about both acoustic and mechanical difllculties.

Other types of receivers have been tried, such as combinations of coupled and tuned circuits, some with fair success in operation, but as a rule such receivers require careful workmanship in construction to make two alike. Further, very often changes in material. through use or change -in temperature so change the characteristics of the unit that it no longer performs the function for which it was designed. Again, where tuned circuits are employed there are always resonant points which continue to persist by virtue of the resonant structure. These are, in many cases, partially smoothed out, but it is never possible to iron them out completely.

Inmy device I have entirely eliminated all resonance indications in the operating range and further have obtained a remarkably sensitive device. This has been possible, not only because of the material which I have used for a diaphragm, but also because of the combinationoi diaphragm and structure, as will appear laier.

In the hydrophone' which I have invented, all the vibrating members are rigid structures and act as levers. Further, these levers are rigidly 5 connected to one anotherjandto the diaphragm in such a. manner "that they all 'must move in the same direction at any one moment. They are, therefore, not free, and depend ,upon thediaphragm entirely. Thus, there is really only one vibratory structure and this is so chosen that it has highviscous losssQyet picks up such an enormous amount of sound energy from the medium that it is a highly sensitive device, as

will appear later.

I have discovered that the diaphragm should be a piston type diaphragm in which practically the entire bending is confined to a small annular section of the diaphragm near its periphery and further that the material should be of the nature of so called linen bakelite; that is, linen sheets impregnated with a phenol resinous product and united in a rigid structure. Such a material presents an extremely rigid element which yields only as a whole and not in spotspas soft rubber might, and further has a high viscous loss which makes the diaphragm practically non-resonant.

An embodiment of the invention is showniin the figure. The 'hydrophone comprises a casing l, which, when used in water, should be watertight and entirely enclosed, a diaphragm 2 and the deand the bolts 6. The diaphragm 2 is composed of so called linen bakelite or the like and is shaped to be thicker at the middle portion than the edges, so 40 that all the bending which takes place is in a small circular strip near the-clamping edge of the diaphragm. At the center of the diaphragm is a stud I, embedded therein and held firmly in place by the nut 8. The stud I has a hole 9 drilled at 5 the top thereof, in which the rod l0 flts. A set screw 1 I, threading in the piece 1 is provided so that the rod Ill might be held firmly in place.

The rod I0 is soldered or welded to a horizontally mounted cross lever i2 between its pivot and its outer edge. At the outer edge of the lever I! there is a rod 13 whichmay be welded or riveted to the lever I! at its end, as indicated in the drawing, and which extends vertically to the armature ll. The armature l4 and the rod I: may

'2 L I zooms-s clamped together by means of a bolt or nut at the end of the rod It. the rod it p through the end of the armature M. or the rod is and the armature I I may be firmly Joined together in any other well-known manner. The armature I 4 is of the balanced type, balanced by the center support It operates in the air gap oi'the permanent magnet is between four poles formed by the magnet. The poles l1 and It may be regarded as north and I8 and 20 as south. When a current is sent in the one direction in the coils 2| and 22, the armature would rock one way and when this curway. This rocking motion, of the armature I4,

which is really a lever, would move longitudinally the rod l3 and ransversely the lever l2. By the action of the lever l2 the rod I0 is, vibrated longitudinally and the diaphragm 2, transversely.

In most cases theapparatus works inversely;

that is, the sound energy is picked up by the diainches, and further is relatively thick at the middie with a bending area near the clamping rim.

The diaphragm is built up of layers of linen impregnated with a phenol resinous condensation product such as bakelite and is, furthermore, very hard and unyielding to local pressure on points on the diaphra but will yield and bend slightly as a whole, the whole yielding and bending taking place at the edge and furnishing, by means oi! the friction between successive layers, very high viscous losses.

The diaphragm, however, being made large this viscous loss is very small as compared with the energy picked up by the diaphragm, so that the sensitivity of the diaphragm is not affected ex- ,45 cent perhaps to cutout the resonant point, which is just what is desired.

As a matter of fact, the exact operation of the diaphragm in this respect is not known, but it is believed that the diaphragmis non-resonant,

. since it is composed and constructed in. such a manner that as the amplitude increases the visccus losses increase very appreciably, so that while the diaphragm has only a small amplitude, the losses are small, but as soon as the amplitude increases, the losses increase and therefore act as a governor of the amplitude of the diaphragm. This feature is believed to be brought about by confining the yielding of the diaphragm to a small area and further having this portion composed of layers of linen which rub against each otheras the diaphragm bends. While, therefore, the diaphragm is sensitive to weak sounds, it builds up in a resistance against strong sounds or resonance.

It should be noted also that none of the lever structures is free at its end and that the diaphragm is so much stiii'er than the pivots or the bending points of the lever system that they cannot possibly resonate at any frequency but must follow exactly the motion of the diaphragm, giving no greater amplitude than the product of the diaphragm amplitude and the mechanical transformer ratio at all frequencies.

To insure against any-freedom of the lever structures, except in their proper motion, not only are they still, but they are mounted in substantially parallel planes and form a parallelogram structure with the connecting rods so that no bending strain is placed on the rods or levers themselves by distortion of the parallelogram structure.

It should be noted that while the levers are rigid they bend easily at their pivots and that therefore no restoring force and no resonance is set up. To insm-e this to afurther degree the diaphragm is so stiii and elastic that whatever restoring force is set up in the lever Joints is very small and practically negligible with respect to that set up in the diaphragm.

Having now described my invention, I claim: 1. A magnetophone for submarine sound reception comprising a watertight casing, a plate of woven textile fabric impregnated with phenol resinous condensation product covering the end of said casing to v an acoustic piston diaphragm having an annular bending section, a magnetophone unit mounted within said casing lever pivoted at one end, said armature and lever being disposed parallel to said diaphragm and perpendicularly disposed means connecting one end of said lever with said armature and consystem is in the diaphragm. 2. A submarine sound receiving device comprising a watertight casing having a diaphragm at one side thereof, said diaphragm being composed of a plurality of layers of relatively thin woven textile fabric impregnated with a phenol resinous condensation product and having an annular bending section with a thicker stifier central portion,'an electromagnetic pick-up means including a movable current inducing element and means connecting the latter with the center of the diaphragm, substantially all of the restoring force for the device being furnished by said diaphragm. 1 3.-A submarine sound receivng device comprising a watertight casing having a diaphragm at one side thereof, said diaphragm being composed of a plurality of layers of relatively thin woven textile fabric impregnated with a. phenol resinous condensation product and having an annular bending section with a thicker stiffer central portion, an electromagnetic pick-up means including a magnet, a pivoted armature associated therewith and means connecting the latter to the center of said diaphragm, substantially .all of] the restoringforce for the device being furnished by said diaphr I r IVAN c. 

